The present invention relates in general to variable valve systems of an internal combustion engine, which have a valve lift degree varying mechanism to vary a lift degree or work angle of engine valves (viz., intake and/or exhaust valves) in accordance with an operation condition of the engine, and more particularly to the variable valve systems of a type that has an actuating mechanism for actuating a control shaft of the valve lift degree varying mechanism.
Hitherto, in the field of variable valve systems, various types of actuating mechanisms for actuating the control shaft of the valve lift degree varying mechanism have been proposed and put into practical use. One of them is shown in U.S. Pat. No. 6,615,777 granted on Sep. 9, 2003.
The actuating mechanism of the U.S. patent generally comprises a threaded shaft that is driven by an electric motor, a screw nut that is operatively engaged with the threaded shaft, a link member that has at one end two arms pivotally connected to diametrically opposed ends of the screw nut through bearing pins, and an adjusting lever member that has one end pivotally connected to the other end of the link member and the other end connected to a control shaft. The control shaft has control or adjusting cams integrally connected thereto.
When, upon energization of the electric motor, the threaded shaft is rotated about its axis, the screw nut is moved axially forward or rearward along the threaded shaft pivotally actuating the link member and the lever member. With this, the control shaft is turned about its axis to a desired angular position.
However, due to its inherent construction, the actuating mechanism of the above-mentioned US patent tends to show the following drawbacks under operation of the engine.
That is, when, because of the biasing force of valve springs that biases intake or exhaust valves in a closing direction, the control shaft is applied with an alternating torque, the adjusting lever member and the link member function to transmit the alternating torque to the screw nut. However, the torque transmission to the screw nut tends to induce a backlash of the screw nut relative to the threaded shaft. Of course, such backlash is undesirable because it induces not only noises of the screw nut but also a premature wear of the threads of the screw nut and the threaded shaft.